This invention relates to a process for producing an aligned carbon nanotube film comprising a multiple of aligned carbon nanotubes. The process of this invention is suitable for producing aligned carbon nanotube films that can be used in many applications including electron emitters, cell electrodes, gas separating membranes, sensors and storage of energy.
The carbon nanotube, first discovered by Sumio Iijima in 1991 [Nature, 354, pp. 56-58 (1991)], is a very slender, hollow tubular carbon material which is generally 1-100 nm in diameter and 1-100 μm long.
It has been proposed with great expectation that the carbon nanotube be used in a wide range of applications including electron emitters, cell electrodes, gas separating membranes, sensors and storage of energy. When carbon nanotubes are to be used in these applications, they are preferably shaped to be aligned in one direction so that they can be assembled into an apparatus effectively and easily while enabling the characters of the individual tubes to be combined in a compact way. It has also been demonstrated that from the viewpoint of electron emitting characteristics and physical properties such as strength, nanotubes with smaller outside diameters are advantageous.
Common methods for producing carbon nanotubes include arc discharge using graphite as an electrode, causing graphite to sublime with a laser, and vapor-phase decomposition of a carbon compound using a suspended metal catalyst. However, many of the carbon nanotubes obtained by these methods have no orientation and it has been impossible to form carbon nanotube bundles or films.
Methods of forming aligned carbon nanotube films or bundles are generally divided into two types, one of arranging preliminarily formed carbon nanotubes on a substrate (see, for example, JP 2001-130904 A) and the other of producing carbon nanotubes on a substrate per se. The second method is more desirable since it makes easy to obtain unidirectional and uniform orientation. Methods of forming carbon nanotubes on a substrate per se include the following: (1) a membrane of a metal which has catalytic ability is generated on a substrate and etched, followed by thermal decomposition of a hydrocarbon on the substrate [WO 00/30141, Chemical Physics Letters, 323, pp. 554-559 (2000), JP 2000-109308 A, JP 2001-15077 A, JP 2001-20071 A and JP 2001-20072 A]; (2) an iron-containing mesoporous silica substrate is prepared by a sol-gel process and reduced with hydrogen, followed by pyrolysis of acetylene [Nature, 394, pp. 631-632 (1998)]; (3) a plasma or microwaves are applied to a substrate to generate carbon nanotubes (JP 2000-203819 A); (4) a thin, single crystal silicon carbide film is formed on a silicon substrate by epitaxial growth, etched away from the substrate and subjected to a high-temperature heat treatment in an oxygen-containing atmosphere (JP 2000-109308 A); and (5) an aluminum substrate is anodized to prepare a template and cobalt is electrochemically deposited into the bottom of the channels in the oxide film and after it is reduced with carbon monoxide, acetylene is thermally decomposed [Applied Physics Letters, 75(3), pp. 367-369 (1999)].
However, these methods have problems such as the cumbersome steps required to prepare substrates for forming aligned carbon nanotubes and the limited area over which aligned carbon nanotubes can be formed and, hence, it has been difficult to realize inexpensive production of large-area, aligned carbon nanotube films that are suitable for the above-listed applications. What is more, those methods can only generate an aligned film of thick carbon nanotubes greater than 20 nm in outside diameter.
There has also been disclosed a method comprising the steps of coating a substrate with catalyst particles, thermally decomposing a hydrocarbon on the substrate and shaving off the generated carbon nanotubes (JP 2001-80912 A) but this process is not capable of producing aligned carbon nanotube films.